Digital photographs may be captured today using a variety of image sensors (e.g., CMOS (complementary metal-oxide semiconductor) image sensors and CCD (charge coupled device) image sensors. Camera functionality is commonly included in today's mobile devices. For example, many cellular telephones such as the Apple® iPhone and the Motorola® Droid include an integrated image sensor that a user may use to capture digital images for transmission or storing on the mobile device. Design of these compact camera system is complicated by the fact that some scenes may exhibit a large degree of contrast (i.e., difference in the degree of intensity between pixels). In other words, within a single scene, some areas of the scene may be well-lit while other areas of the scene are masked in shadow. For example, when a user takes a picture outdoors, the scene may contain some objects in direct sunlight and other objects that are shielded from the sun.
CMOS image sensors used in mobile devices have a limited dynamic range. Each pixel site in the CMOS image sensor functions like a capacitor, capturing photons focused on the image sensor by a lens during an exposure and building up a charge. The amount of charge developed at a particular pixel site is dependent on the well-capacity of the pixel sensor. For example, CMOS pixels approximately 1.4 μm in size have a well-capacity of approximately 5000 electrons. Once the pixel site has built up a charge equivalent to 5000 electrons, the pixel site is incapable of capturing any further information about the brightness of the scene. The upper limit of the dynamic range is governed by the well capacity, and the discrete nature of light. Shot-noise limits the highest signal-to-noise ratio (SNR) of the sensor to the square root of the maximum signal, or about 36 dB in our 5000 electron example. The lower limit of the dynamic range is governed by read noise and quantization. Even in the absence of read-noise, the charge on the pixel is sampled to a discrete digital value; e.g., a 10-bit value. The charge for a pixel may be digitized using a 10-bit ADC (analog-to-digital converter) to generate a value between 0 and 1023.
As described above, the image sensor is only capable of measuring a limited dynamic range of light. Thus, the information captured by the image sensor is dependent on the exposure time. Using a fast exposure time may prevent bright areas of the scene from saturating the corresponding pixel sites. However, detailed information in darker areas of the scene may be lost because the signal in these areas is weak. Conversely, by extending the exposure time, details in the darker areas of the scene may become visible, but the brighter areas of the scene may become overexposed.
One technique for generating images with high-dynamic range (HDR) is to capture two images of the same scene using different exposure times. Conventionally, a first image is captured with one exposure time and then a second image is captured with a second exposure time. Once the images are captured, an image processing pipeline combines the two images to generate a scene with a dynamic range that is larger than the image sensor is capable of capturing during a single exposure. Recently, interleaved image sensors have been developed that capture two images with different exposure times substantially simultaneously. In effect, the interleaved image sensor captures one image of the scene using two different but simultaneous exposure times interleaved throughout the image sensor.
Some image processing algorithms for generating images using interleaved image sensors sacrifice spatial resolution to generate HDR images. For example, a first image may be generated using half the pixels and a second image may be generated using the other half of the pixels. The first image and the second image are then blended to generate an HDR image at half the vertical resolution. Thus, there is a need for addressing this issue and/or other issues associated with the prior art.